ARDS s a syndrome with a 50% mortality. Current therapy for ARDS is treatment of the underlying precipitating disease or event, supportive care and avoiding complications. Patients are usually managed in the supine position, but there are several case series that suggest the prone position may improve oxygenation in mechanically ventilated (MV) patients with ARDS (78% out of 87 patients had improvement in oxygenation). This may be due to changes in regional lung inflation, redistribution of perfusion or changes in transpulmonary pressure gradients. Baseline data is recorded. The patient is then turned to the prone position and arterial blood gases are done twice post turning as well as collection of pulmonary artery (PA) catheter and MV data. The MV may be adjusted and the patient returned to the supine position if needed to maintain oxygenation. Patients who desaturate will be returned to the supine position and retried in the prone position 24 hours later. Patients who have improvement will remain in the prone position for 24 hours at which time they will be returned to the supine position. Blood gases and data collection will be done 4 times in the prone position and 2 times in the supine position. Other supportative care continues, including repositioning every 2 hours. If oxygenation worsens or does not improve the patient exits the study. The patient also exits the study if the investigator thinks the prone position is compromising care, dialysis or vasopressors are needed or a tracheostomy is done.